They're hardly your typical jingle singers, but the African a cappella group Mukulu hit the right note with Dannon Yogurt. Dannon's rep at Grey Advertising, Richard Mendoza, received a tape of the 10-member Ugandan group and decided that its smooth harmonies went well with the company's new fat- free ''Dannon blended.'' Mukulu's commercial for Dannon broke in the spring in the Northeast and will begin airing in other markets this fall. Alex Mukulu, the group's 38-year-old leader, who engineered its small U.S. tour earlier this year, is looking to tour again this time as the opening act for Amy Grant. He's also negotiating for an American record contract, with help from manager Bob Farnsworth of Nashville's Hummingbird Productions. Of course, in live performances the group will sing its more usual ''Bukedde bulungi'' (''It's dawn, a good dawn'') rather than the ''smooth and white, creamy, cool'' sentiments it mouths for Dannon. Although only a few of Mukulu's songs are in English, Farnsworth thinks audiences will get the message anyway. ''Their music is detailed and intricate but really upbeat,'' he says. ''When you listen to it, you think that all is right with the world.''